Electroencephalograph

Abstract

A multi-channel instrumentation system is provided wherein signals from selected electrode pairs are sequentially connected to the isolated input of a common signal amplifying path. The electrode pair selecting means is included in a portable head box which may be placed adjacent to the patient to be tested. Included in the head box are facilities for testing the impedance of any selected one of the patient electrodes and providing a visual indication of the magnitude of such impedance. The head box also includes facilities for selecting a group of patient electrodes which are connected together and used as an average or reference potential for one input of the common signal path. Facilities are also provided for calibrating the common signal path by applying a d.c. calibration signal to the input thereof by means isolated from sytem ground. An improved circuit arrangement is provided for limiting the current which can be drawn from the patient ground electrode to a small value to provide for the safety of the patient in the event of malfunctioning of the electrode selecting means. Facilities are provided in the common signal path for removing the d.c. components of the electrode pair signal voltages, and for selectively varying the gain of the common signal path on an individual channel basis.

What is claimed as new and desired to be secured by Letters Patent of the United States is: 
     
       1. In an EEG system, a plurality of patient electrodes, a differential amplifier having first and second inputs, means for selectively connecting a selected one of said electrodes to said first input and simultaneously connecting a selected one of said electrodes to said second input during each of successive time periods which recur in repetitive time position frames, means for reversing the polarity of the output of said differential amplifier during successive ones of said time periods in a time position frame, recording means having a plurality of input channels corresponding to said successive time periods, successive ones of said input channels being arranged to respond to oppositely polarized input signals by producing traces in the same direction, and demultiplexing means for sequentially connecting said reversed polarity amplifier output to said plurality of recording input channels. 
     
     
       2. The EEG system of claim 1, which includes a capacitor connected to each of said input channels, said demultiplexing means connecting the reversed polarity output of said differential amplifier to each of said capacitors in sequence during said time periods. 
     
     
       3. The EEG system of claim 2, which includes a resistor common to said capacitors and forming a low-pass filter with each of said capacitors. 
     
     
       4. The EEG system of claim 3, which includes means for varying the value of said common resistor, thereby to vary the frequency characteristic of all of said low-pass filters while maintaining the polarity reversal of adjacent ones of said input channels.